Still the by far largest problems for the hearing impaired is to understand speech in the presence of competing sources. This stems from the lost ability to segregate sources well enough to be able to “tune in” on the currently most interesting signals. Normal-hearing persons has this ability to segregate and tune in, usually referred to as the cocktail party effect. The brain and the auditory system do this tuning by adjusting the neural gain towards the sources of interest. Recent brain-imaging data shows that the typical neural gain is in the order of 10-12 dB. This means that the brain neurologically can emphasize a source of interest by an amount of 10-12 dB. This neural gain is much lower for the hearing impaired, it is close to 0 dB, which explains why the hearing impaired complains that sounds tend to fuse, that is they can no longer be segregated and tuned in to by the hearing impaired.
A directional microphone/beamformer works by suppressing sounds arriving from some directions, while maintaining good sensitivity from one direction (usually the front). The ‘opposite’ of a directional microphone is an omni-directional microphone, which has the same sensitivity towards sound from all directions. A first-order subtractive microphone usually implements the directional microphone in a hearing aid, where the output from two omni-directional microphones are subtracted. The sound recorded at the rear microphone is time-delayed and then subtracted from the frontal microphone. The time it takes for the sound to travel between the microphones is called the external delay, and it is calculated as the distance between the microphones divided by the speed of sound. The delay that is introduced electrically inside the hearing aid is called the internal delay. If the sound arrives from the rear and the internal delay is equal to the external delay, the front and delayed rear signals will cancel out each other and the hearing aid will thus be insensitive to sounds arriving from the rear. The ratio between the internal and external delay, called the delay ratio, determines the sensitivity to sounds arriving from different directions. The sensitivity pattern of a directional microphone is usually indicated using a polar plot. The polar plot reveals the microphones sensitivity to sound arriving from different directions.
The above argumentation indicates that the current beamforming technology, even with binaural beam formers, does not fulfill the necessary 10-12 dB segregation under reverberant conditions to re-establish the hearing impaired persons lost segregation abilities. Furthermore, the beams are too wide (30+ degrees) to be really selective. Other configurations with narrower beams exist, but with increased side lobes from the back.
A number of remote microphones placed close to the talker and using for example eye-gaze to let the user intuitively control which talker to tune in to can in principle solve the problem.